Pre-lab: Earthworm Dissection
1. The term annelida means:
2. The textbook states that “Annelids are worms with segmented bodies. They
have a true coelom that is lined with mesoderm.” What is a coelom?
Give the function of the following terms:
Parts located on the outside of the worm: (Give the function of each)
Clitellum:
Female Genital Pore:
Male genital pore:
Mouth:
Prostomium:
Setae:
Parts located on the inside of the worm (give the function of each)
Crop:
Dorsal Blood vessel:
Esophagus:
Gizzard:
Intestine:
Mouth:
Nephridia:
Pharynx:
Septum:
Name:
Lab: Earthworm Dissection
Background: Among the most familiar invertebrate animals are the earthworms,
which are members of the phylum annelida. The word annelida means “ringed”
and refers to a series of rings or segments that make up the bodies of the
members of this phylum. Internally, septa, or dividing walls are located between
the segments. There may be more than 100 segments in an adults worm.
The clitellum is a swelling of the body found in sexually mature worms and is
active in the formation of an egg capsule, or cocoon. Eggs are produced in the
ovaries and pass out of the body through female genital pores. Sperm are
produced in the testes and pass out through tiny male genital pores. During
mating, sperm from one worm travel along the sperm grooves to the seminal
receptacle of another. Fertilization of the eggs takes place outside the body as
the cocoon moves forward over the body, picking up the eggs of one worm and
the sperm of its mate.
The pumping organs of the circulatory system are five aortic arches. Circulatory
fluids travel from the arches through the ventral blood vessel to capillary
beds in the body. The fluids then collect in the dorsal blood vessel and
reenters the aortic arches.
The earthworm takes in a mixture of soil and organic matter through its mouth,
which is the beginning of the digestive tract. The mixture enters the pharynx,
which is located in segments 1 – 6 . The esophagus, in segments 6 – 13, acts
as a passageway between the pharynx and the crop. The crop stores food
temporarily. The mixture that the earthworm ingests is ground up in the gizzard.
In the intestine, which extends over two-thirds of the body length, digestion and
absorption takes place. Soil particles and undigested organic matter pass out of
the worm through the rectum and anus.
The nervous system consists of the ventral nerve cord, which travels the length
of the worm on the ventral side, and a series of ganglia, which are masses of
tissue containing may nerve cells. The nerve collar surrounds the pharynx and
consists of ganglia above and below the pharynx. Nervous impulses are
responsible for movement and responses to stimuli. Each segment contains an
enlargement, or ganglion along the ventral nerve cord.
Excretory functions are carried on by nephridia, which are found in pairs in each
body segment. They appear as tiny white fibers on the dorsal body wall. The
earthworm has no gills or lungs. Gases are exchanged between the circulatory
system and the environment through the moist skin.
Purpose: Describe the appearance of various organs found in the earthworm.
Locate, name and give the function of these organs.
Materials: Dissecting pins
Dissection tray
Scalpel
Paper towel
Forceps
Teasing needle
Preserved worm
Scissors
Hand lens
Procedure:
1. Place a damp paper towel in the bottom of a dissection tray. Place an
earthworm on the towel, identifying the dorsal, ventral, posterior and anterior
regions. The dorsal side is the worm’s rounded top and the ventral side is its
flattened bottom.
Figure 1 - 1
Outside of the worm:
3. Use a hand lens to observe and locate all the parts of the worm externally
using figure 1-1.
a. Locate the prostomium, which is the fleshy lobe that extends over the
mouth on the anterior end of the worm. The other end of the worm’s body
is the posterior end, where the anus is located.
b. Locate the clitellum, which extend from segment 33 to segment 37. Look
for the worm’s setae, which are the minute bristlelike spines located on
every segment except the first and last one.
c. Using figure 1 – 1, locate and identify the external parts of its reproductive
system. Find the pair of sperm grooves that extend from the clitellum to
about segment 15, where one pair of male genital pores is located. Look
also for one pair of female genital pores on segment 14. There is another
pair of male genital pores on about segment 26. Try to find the two pairs of
openings of the seminal receptacle on segment 10. Note: These openings
are not easy to see.
4. Moisten worm to keep it from drying out. Then turn worm dorsal side up.
Inside of the worm:
5. Slightly stretch out the earthworm’s body. With dissecting pins, pin the first
and last segments of the earthworm to the bottom of the dissecting tray.
Locate the dorsal blood vessel, a dark line that runs along the midline of the
dorsal surface. The vessel runs from the anterior end to the posterior end.
Insert the tip of a pair of scissors just to the right of the dorsal blood vessel and
about 10 segments from the posterior end. Cut along the blood vessel to the
anterior end. Keep the scissors parallel to the bottom of the dissecting tray.
Keep the cut as shallow as possible. The body wall of the earthworm is very
thin, and the internal organs lie just inside. Note: The major internal organs of
the earthworm are anterior to the clitellum. Be very careful when cutting in this
area.
6. When the cut is complete, carefully open the body wall. Notice that the
coelom is not one large cavity but is separated into segments by septa
(singular, septum), or thin partitions of tissue. The septa are continuations of
the external segments. Beginning at the anterior end, use a probe or
dissecting pin to carefully tear the septa from the anterior to the posterior end.
Place dissecting pins to hold open the body wall. Angle the pins outwards.
Then place pins in every tenth segment, to help you locate the major internal
organs. Use eyedropper to moisten the insides of the earthworm.
7. Trace the digestive system of the earthworm from the mouth to the anus. See
figure 1-2.
a. The mouth is located in the first three segments. Locate a slight swelling,
the muscular-walled pharynx posterior to the mouth in segment 3 to 6.
b. The slender esophagus, located in segments 6 to 14, empties into the thinwalled crop, located in segments 15 and 16. The crop temporarily stores
food.
c. The gizzard is located in segments 17 and 18 and is a grinding organ. It
mixes food with sand from the soil and physically breaks the food into
smaller pieces. Use the probe to feel the difference in the walls of the
crop and the gizzard.
d. The intestine or straight tube leads from the gizzard to the anus.
Figure 1-2
8. Circulatory system:
a. Trace the earthworm’s closed circulatory system by following the dorsal
blood vessel anteriorly along the midline to the five aortic arches, located in
segments 7 to 11.
b. In the posterior section use a probe to carefully move aside the intestine.
Locate the ventral blood vessel. The aortic arches connect the dorsal
blood vessel with the ventral blood vessel.
Circulatory System of the Earthworm:
Systems of the worm
9. Locate the brain, which is a white mass of tissue found in the third segment
anterior and dorsal to the pharynx.
a. With a probe, gently push aside the pharynx and esophagus to expose the
white ventral nerve cord. The ventral nerve cord runs along the inner
ventral surface from segment 3 to the last segment.
b. With a hand lens, you may be able to see the two-branched nerves
that run from the brain around the esophagus to the ventral nerve cord.
c. In each segment, locate the mass of nerve cell, or ganglia, attached to the
ventral nerve cord.
10. With a probe, push aside the intestine. Use a hand lens to look for the white,
coiled nephridia. There is a pair of nephridia in every segment except the
first three segments and the last segment. Each nephridium collects wastes
from the coelom and carries them to the outside of the body through pores or
openings, in the body wall of each segment.
11. An earthworm has both male and female reproductive organs and can
produce both eggs and sperm. Use a hand lens during the remainder of the
dissection. Please note, with the exception of the seminal vesicles, most of
the reproductive organs are small and difficult to see in preserved specimens.
a. First locate the male reproductive organs. Find the two seminal vesicles in
segments 9 and 13. The seminal vesicles are three-lobed, light-colored
organs along the side of the esophagus.
b. Two small pairs of testes are located in segments 10 and 11.
c. The female reproductive organs include a pair of small, spherical seminal
receptacles, located in segment 9 and 10, and a pair of ovaries, located in
the septum between segments 12 and 13.
Post Lab Questions:
1. Describe the shape of the dorsal and ventral surfaces of the earthworm’s
body.
2. Which segments is the clitellum located?
3. Give the body position of the setae on the earthworm?
4. Compare the walls of the crops and the gizzard and give the function of each.
Crop:
Gizzard:
5. Trace the parts of the digestive tract through which food passes. Include all
parts.
Analysis and Conclusions:
1. How is the earthworm’s digestive system adapted for extracting relatively
small amounts of food from large amounts of ingested soil?
2. Why is it important not to make a deep cut with the scissors when dissecting
your earthworm specimen?
3. Describe two ways in which an earthworm’s body is adapted to life in the soil.
4. Describe one way in which an earthworm is poorly adapted to life on land.
5. How might an earthworm’s lack of appendages be an adaptation to
burrowing?
6. Explain how an earthworm enriches and aerates the soil, thus improving it for
plant growth.
Exterior:
Circulatory System of the Earthworm:
Systems of the worm
Earthworm
Name:
Hour:
Directions: Read lab and answer the following questions.